CHAPTER_III - Blank Holder Force

7/29/2019 CHAPTER_III - Blank Holder Force

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CHAPTER 3

EXPERIMENTAL STUDIES

To prove the feasibility of the process and to get a clear picture on the

processes, exhaustive experimental studies were conducted. A 100 ton double

action hydraulic press with required tooling was designed and fabricated for

this purpose. Necessary instruments were installed onto the press to capture

the vital process parameters. The whole unit was controlled by aprogrammable logic controller.

In order to comparatively estimate the prospects of the newly

developed processes, experiments were also conducted on conventional deep

drawing process. Initially important process variables affecting all the three

processes were sorted out. Maximum LDR that could be obtained through the

processes was found. Taguchis experimental technique with ANOVA was

used to identify the significant process variables affecting wrinkling, thinning

and maximum punch force requirements in each of the processes. After

identification, full factorial experiments were conducted and the results were

acquired.

3.1 Experimental setup

The base unit is being a 100 ton double action hydraulic press, the

blank holder setup and the die sets were mounted on the blank holding slide

and bottom platen respectively as shown in Figure 3.1.

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Figure 3.1 Line diagram of the experimental Setup

3.1.1 Hydraulic press

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The specification of the double action hydraulic press used is given in

the Table 3.1.

Table 3.1 Hydraulic press specifications

Provision was given to vary the main slide velocity within the stroke.

The press was controlled by a programmable logic controller and has both

auto and manual modes. T-slots were machined in both the slides and in

bottom platen to mount the required toolings.

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Parameter Value

Total capacity 100 tons

Main ram 50 tons

Blank holding ram 20 tons

Die cushioning 30 tons

Platen size 900x500mmDay light 750 mm

Main slide stroke 400 mm

Blank holding slide stroke 400 mmMain slide velocity range 1-35 mm/sec


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3.1.2 Die set

Die sets were mounted onto the bottom platen and it comprises of a die

block, a die insert and a strip ring. Provision was given in the die block and

the die insert to mount the pressure relief valve. The die set is depicted in the

Figure 3.2.

3.1.2.1 Die block

The die block was made of mild steel and the outer and inner diameter

were 300 mm and 70 mm respectively. Grooves were machined to the bottom

and the top surfaces of the block. Die insert was screwed in the bottom groove

through grab screws. Groove in the top surface was used to position the

locating ring.

3.1.2.2 Die inserts

The material used for die inserts was high carbon high chromium steel.

Three numbers of die inserts were used with corner radii 3, 5 and 7mm

respectively. The outer diameter of the die inserts was being 70mm and the

inner diameter was 51.8 mm. Punch-die clearance was chosen to be 1.2t,

where t is the thickness of the blank.

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Figure 3.2 Die set


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3.1.2.3 Locating rings

Locating ring was used to locate the blank in the die set. Four numbers

of locating rings were used. The outer diameter of the locating rings was the

diameter of the groove in the top surface of the die block and the inner

diameter varies from 75 mm to 150 mm. This accommodates the blank of

various diameters.

3.1.2.4 Blank holding unit

Since the current research focuses mainly on blank holding, the blank

holding unit was carefully designed and fabricated. The important parts of the

blank holding unit were the lubricant container, the rotating shaft, the pad

holder, the pressure pad and an external power pack. The blank holding unit

was shown in the Figure 3.3.

Lubricant container

As the name itself suggests it is just a container made of C45 material, used to

store and supply the lubricant to the pressure pad during the process. The

lubricant container was mounted to the blank holding slide of the hydraulic

press through a rectangular plate.

Shaft

The shaft was mounted to the lubricant container through an inclined groove

ball bearing. The shaft is used to transmit the power to the pressure pad from

the variable speed drive. The shaft consists of four holes of 4 mm diameter

through which the lubricant oil is supplied to the pressure pad. Provision is

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also given to the shaft to cut down the lubricant supply during ideal

conditions. C45 material was used for the shaft.

Pad holder

The pad holder was also made of C45 material. It is just a cylindrical plate

fastened to the shaft which is used to mount the pressure pad.

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4

Figure 3.3 Blank holding unit


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Pressure pad

Twelve numbers of pressure pads were produced with various outer

diameters ranging from 75 to 150 mm and with different radial distances of thecircumferential holes. The pressure pad was made of high carbon high chromium

steel and was hardened to resist wear. The surface finish in the contact interface

was Ra 3 . Oil seals were provided in the blank holding unit at the required places

to control the leakage of the lubricant and to arrest the flow of lubricant to the

punch region

3.1.2.5 Punch

The diameter of the punch used was 50mm. Three punches with corner

radii 3, 5 and 7 mm were used in the experiments. The punches were made up of

high chromium high carbon steel and were hardened and tempered. The punch

was mounted to the main slide.

The images of the different parts of the experimental setup are given in

APPENDIX 3

3.1.2.6 Lubricant

Deep drawing lubricants with different viscosities were used. Deep

drawing lubricant designated as SHELL FENELLA FLUID DS 2240 was used for

die-blank interfaces. For the pressure generation purposes in the blank-blank

holder interfaces SHEL FENELLA FLUIDS CH 401 and CH 402 were used. The

properties obtained from the supplier are as listed in Table 3.2. Molybdenum

disulphide powders designated as Molykote Z and Molykote microsize are used

for the process with MoS2 lubrication. The properties are listed in Table 3.3.

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Table 3.2 Fluid Lubricant properties

PropertyValue

DS 2240 CH 401 CH 402

Viscosity @ 100C 1100 SUS 10.7 cSt 17.45 cSt

Viscosity @ 40C - 78.3 cSt 154 cStFilm property Tacky, stiff Tacky, adhesive Tacky, adhesive

Table 3.3 Molykote Properties

PropertyMolykote Powder

Z Powder Microsize Powder

Density at 20C (g/ml) 4.80 4.80

Coefficient of friction 0.05 0.06

Particle size (m) 3-4 0.65-0.75

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3.1.3 Instrumentation

In order to quantify the required process parameters online and offline

necessary instruments were used. The Table 3.4 lists the vital process parameters

along with the instruments used for their measurements. All the instruments were

calibrated and are connected to the data logger of the PLC unit. The software of

the PLC unit has the capability to plot all the measured parameters with respect to

time and punch stroke.

Table 3.4 Instrumentation

Parameter Instrument

Punch force and blank holding force Pressure transducers of 300 bar capacity

and 0.1 % accuracy

Punch stroke and velocity Linear variable displacement transducer

of 200 mm stroke and 0.1% accuracy

Hydrostatic pressure 6 numbers of miniature pressure sensors

of capacity 0-10 bar and 0.1% accuracy.

3.2 Blank material and preparation

Commercial quality low carbon steel of 0.7 mm thickness was used as

blank material. The composition of which is furnished in Table 3.5.

Tests were conducted to ascertain the mechanical properties of the material.

The important properties measured were the tensile strength, yield strength, strain

hardening exponent and plastic strain ratio.

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Table 3.5 Blank material composition

Material % Composition

Carbon 0.06

Manganese 0.5

Silicon 0.065Sulphur 0.009

Phosphorous 0.013

Rapid n test was conducted to find the strain hardening exponent. Test

specimen was prepared as per the given size depicted in Figure 3.4. Nearly 12

specimens on the different direction of sheets were tested and the resulting

thickness strain and width strain are measured for 20% elongation. Table 3.6 lists

the properties and Table 3.7 lists the data acquired from the experiments.

Table 3.6 Blank material properties

Property Value

Normal anisotropic parameter (rm) 1.06

Planar anisotropic parameter (r) 0.035

Strain hardening exponent (n) 0.21

Tensile strength 338.4 N/mm2

The blanks were prepared at different sizes ranging from 80 mm to

150 mm and their edges were trimmed. The thickness tolerance and the surface

roughness of the blanks were measured and were found to be around + 0.5% of

Nominal Thickness and Ra 0.5 respectively.

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All dimensions are in mm.

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Figure 3.4 Test specimen


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Table 3.7 Blank material properties - Data acquired from experiments

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3.3 Methodology adopted

The following section elaborates the methodology adopted during the

experimental studies.

3.3.1 Conventional deep drawing process

For the conventional deep drawing process the lubricant FENELLA

FLUID 2240 was wiped on to both the side of the blank. The die block was

assembled with the required die insert and locating ring and was placed over

the bottom platen. The punch and die block was checked for the eccentricity

and was aligned. The blank is placed over the surface of the die block and was

held in position by the locating ring.

4

o.

Sheet

direction

Thicknes

s strain

Width

strain

Plastic

strainratio (r)

Strain

hardening

exponent

(n)

(r)

(Avg.)

(n)

(Avg.)

Cross

sectionalarea

(mm2)

Load

onfailur

e (N)

Tensile

strength(N/mm2)

0

-0.168 -0.210 1.430 0.22

1.513 0.217

4.295 1456 339

-0.163 -0.239 1.470 0.21 4.240 1434 338

-0.153 -0.220 1.440 0.23 4.315 1468 340

-0.151 -0.188 1.712 0.21 4.331 1501 346

45

-0.186 -0.170 0.913 0.19

1.044 0.200

4.392 1427 324

-0.162 -0.177 1.090 0.20 4.397 1439 327

-0.185 -0.180 0.973 0.21 4.295 1490 346

-0.143 -0.172 1.200 0.20 4.310 1510 350

90

-0.250 -0.159 0.636 0.21

0.646 0.210

4.331 1435 331

0 -0.220 -0.154 0.701 0.22 4.237 1440 339

-0.280 -0.158 0.561 0.21 4.335 1467 338

2 -0.219 -0.151 0.689 0.20 4.301 1478 343


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The pressure pad with a plane surface was used for this process. The

pressure pad was fitted to the blank holding unit and the blank holding slide

was brought down and the holding force was applied to the blank through the

pressure pad. After ascertaining that the applied blank holding force had

reached the preset magnitude, the punch slide was brought down and the

drawing was started. The punch stroke and the velocity were fixed prior to the

process.

The punch force throughout the stroke was measured with the help of a

pressure transducer mounted on the fluid line of the main cylinder. The values

were later plotted with respect to the punch stroke. The punch velocity and the

stroke could be captured online using LVDT. Once the cup was drawn

successfully, the punch was retracted and the pressure pad is lifted. The cup

was ejected using ejector fitted to the die.

3.3.1.1 Maximum LDR

The studies aimed in estimating the maximum limiting draw ratio that

could be obtained through the conventional process for the chosen material.

The vital process parameters of the conventional process are listed below

Punch corner radius

Die corner radius

Punch velocity

Blank holding force

Punch - die clearance

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Failure modes in deep drawing is of two kinds, the primary one is due to

excessive plastic deformation at the punch profile region which results in

thinning and tearing. The secondary one is being wrinkling. A cup which is

successfully formed should be free from these two defects. The magnitude of

the process variables chosen to estimate the maximum limiting draw ratio was

purely by trial and error method based on the literature survey conducted. No

definite experimental technique was used for this purpose. But for undergoing

the studies on the failure modes such as thinning and wrinkling, design of

experiment techniques were applied.

The Table 3.8 gives ranges of various process variables that could be

applied with the designed experimental setup. The table also lists the values

used when estimating the maximum limiting draw ratio.

The magnitude of the process variables chosen to estimate the maximum

LDR was based on the recommendations given in Lange (1985). The punch

and die corner radii was held maximum since the literature clearly suggests

that larger corner radii aides higher draw ratio. The punch velocity doesnt

have much influence on the draw ratio and hence it was fixed as 10 mm/sec

which is well below the upper limit for given material. Similarly punch-die

clearance also does not have much influence on the draw ratio, even though

the value was held at its maximum limit to prevent ironing and burnishing of

blank during drawing.

LDD= D-1.25d (3.1)

Where LDD is the limiting draw depth, D is the diameter of the blank

and d is the diameter of the punch. But it was decided to keep the punch

stroke below the limiting draw depth to retain the flange area.

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Table 3.8 Process variables used for estimating maximum LDR

Process variable Range Used values

Punch corner radius 3,5,7 mm 7 mm

Die corner radius 3,5,7 mm 7 mm

Punch velocity 1-35 mm/sec 10 mm/sec

Blank holding force 0 to 200 KN Calculated using Eqn. 3.2

Punch-die clearance 1.58 mm 1.58 mm

Lubricant DS 2240, CH 401, CH 402 DS 2240

The only variable that should be varied during experimentation is the

blank holding force, since there is no definite rule to fix the magnitude of the

same. The empirical relation given as Equation 3.2 approximately estimates

the range of blank holding force that could be applied to hold the blank for the

given material and for the given diameter of the blank.

( )( )uobh

tdLDRDP /005.01/1103

3+=

(3.2)

Where Pbh is the blank holding pressure, D is a factor ranging from 2 to

3, d is the blank diameter, to is the blank thickness and u is the ultimate

tensile strength of the material.

In order to reduce the number of experiments to be conducted, it was

decided to initially predict the maximum LDR through the valid theoretical

relation that was already derived. One such relation is given by Leu (1997) as

LDR= 121

22

12

+

++ rnrfeee

n

(3.3)

where n is the strain hardening exponent, r is the normal anisotropic

parameter and f is the drawing efficiency.

By assuming 80% drawing efficiency the maximum LDR predicted

through the above relation was 2.08 and hence it was decided to start with 2.1.

If the cup with LDR 2.1 could be drawn successfully it was decided to go

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above 2.1 or else to come below 2.1. Each experiment was conducted thrice.

A sample data sheet used during experimentation is given in Figure 3.5. For

the limiting draw ratio of 2.1 the blank holding force was applied within the

range of 3.55 KN to 5.34 KN. Experiments have been conducted for three

levels within the range.

The failure modes during drawing were observed and the punch stroke

during failure was measured. If the failure was due to excessive thinning and

fracture then the magnitude of the blank holding force was reduced and if the

failure was due to wrinkling then the magnitude of the blank holding force

was increased. The experiments were repeated again for lesser or higher blank

holding force. If the failures were not observed then the cup was said to be

drawn successfully and required LDR is achieved.

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3.3.1.2 Thinning studies

The drawability of a metal depends on two factors:

The ability of the material in the flange region to flow easily in the

plane of the sheet under shear.

The ability of the sidewall material to resist deformation in the

thickness direction.

The punch prevents sidewall material from changing dimensions in the

circumferential direction; therefore, the only way the sidewall material can

flow is by elongation and thinning. Thus the ability of the sidewall material to

withstand the load imposed by drawing down the flange is determined by its

resistance to thinning, and high flow strength in the thickness direction of the

sheet is desirable. Taking both of these factors into account, it is desirable in

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unch corner radius - 7 mm Punch- die clearance - 1.58mm

ie corner radius - 7 mm Punch velocity - 10mm/sec

xpt

No

Blank

holding

force (KN)

Sample No.

Punch stroke at failureMaximum

punch force

Minimum

sectional

thickness

RemarksThinning Wrinkling

3.55 KN

1

2

3

4.44 KN

1

2

3

5.34 KN

1

2

3

Figure 3.5 Sample data sheet - Estimation of Maximum LDR -

Conventional deep drawing Process


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drawing operations to maximize material flow in the plane of the sheet and to

maximize resistance to thinning in the direction perpendicular to the plane of

the sheet.

Though maximizing the resistance to thinning greatly depends on the

material variables, the other approach to reduce the thinning is to reduce the

load carried by the sidewall. This could be well achieved by choosing the

optimum values of the process variables. By doing so it is also possible to

maintain almost uniform wall thickness throughout the height of the cup.

The significant process variables responsible for thinning are to be

identified and optimum values of those variables found. The study was carried

out at the maximum limiting draw ratio obtained through conventional deep

drawing process.

Design of experiments

Experimental design is used to identify or screen important factors

affecting the process, and develop empirical methods of those processes.

Design of experiment techniques enable the user to learn about process

behaviour by running a series of experiments, where a maximum amount of

information can be obtained, in a minimum number of runs. Tradeoffs as to

amount of information gained for the number of runs undertaken are known

before running the experiments.

Experimental design based on orthogonal arrays was made popular by

the Japanese engineer Genechi Taguchi. They are usually identified with the

name such as L8, to indicate an array with eight runs. A Taguchi L 8 array

shown in Table 3.9 is used to investigate the effects of up to seven factors in

eight runs.

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Two such works in the same field have already been reported. Browne

et al (2003) used L8 orthogonal principle along with ANOVA to optimize the

variables when deep drawing CR 1 cylindrical cups. Similarly Mark Colgan et

al (2003) used it for optimizing the draw force and thickness distribution.

For each design, each row represents runs of the experiment; here each

design has eight runs. Each column represents the settings of the factor at the

top of the column. In the Taguchi design, the levels are (1, 2) each means

(low, high) for each factor.

For a fall factorial design, the number of possible designs N is

N=Lm (3.4)

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5

Expt. No.Factor number

1 2 3 4 5 6 7

1 1 1 1 1 1 1 1

2 1 1 1 2 2 2 23 1 2 2 1 1 2 2

4 1 2 2 2 2 1 1

5 2 1 2 1 2 1 2

6 2 1 2 2 1 2 1

7 2 2 1 1 2 2 1

8 2 2 1 2 1 1 2

Table 3.9 L8 Orthogonal


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where L is the number of levels for each factor and m the number of factors.

Thus a full factorial for the parameters of an L8 (27) would consist of 128

experiments.

Hence for conducting screening experiments Taguchis L8 orthogonal

array is used, by which 7 different factors are analyzed for their effects on

thinning, wrinkling and maximum punch force requirements. The analysis for

identifying the significant factors is done using analysis of variance

(ANOVA).

ANOVA was developed by Sir Ronald Fisher in the 1930s as a way to

interpret the results from agricultural experiments. ANOVA is not a

complicated method and has a lot of mathematical beauty associated with it.

ANOVA is statistically based, objective decision making tool for detecting

any differences in average performance of groups of factors tested. The

decision, rather than using pure judgment, takes variation into account.

Table 3.10 shows the L8 orthogonal array with the experimental factors

to be varied. Table 3.11 shows the parameters and the levels that have been

decided upon to use. Each experiment was conducted three times. With regard

to thickness, the standard deviations of the thickness values measured along

the wall of the cup were taken for use in the ANOVA.

For measuring the thickness the cup was sectioned at the mid point to

unveil the wall thickness throughout. The sectioned wall was polished and

placed under the microscope and the thickness measurements were taken at

fixed points along the wall. The minimum wall thickness for all the cups

formed was found and the analysis of variance was conducted over the

results.

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Table 3.10 Orthogonal array along with factors - Conventional deep drawing process

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Table 3.11 Levels of factors used Conventional deep drawing process

Punch corner

radius (mm)

Die corner

radius (mm)

Blank holder

force (N)

Lubricant

type

Lubricant

position

Punch

velocity

mm/sec

Draw depth

1 Low 3 3 a* DS 2240 Die 5 c*2 High 7 7 b* CH 402 Punch-die 15 d*

a* - lower range of blank holding force calculated with Equation 3.2 for the given LDR

b* - higher range of blank holding force calculated with Equation 3.2 for the given LDR

c* - 50% of the limiting draw depth calculated using Equation 3.1 for the given LDR

5

Expt.

No.

Factor number

1 2 3 4 5 6 7

Punch corner

radius (mm)

Die corner

radius (mm)

Blank holder

force (N)

Lubricant

type

Lubricant

position

Punch velocity

mm/sec

Draw depth

1 1 1 1 1 1 1 1

2 1 1 1 2 2 2 2

3 1 2 2 1 1 2 2

4 1 2 2 2 2 1 1

5 2 1 2 1 2 1 2

6 2 1 2 2 1 2 1

7 2 2 1 1 2 2 1

8 2 2 1 2 1 1 2


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d* - 75% of the limiting draw depth calculated using Equation 3.1 for the given LDR

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The % significance of each factor with respect to the thickness

distribution was obtained. After obtaining the significant parameters; keeping

all other parameters constant, the top three parameters were varied at multiple

levels and full factorial experiments were conducted.

3.3.1.3 Wrinkling studies

Wrinkles can limit the ability to stretch sheet metal during processing

and adversely affect final product appearance, assembly and functionality.

Severe wrinkles may damage or even destroy dies. In typical drawing process,

a restraining force is applied through the blank holder and/or through

bending/unbending force imposed by drawbeads. Such a restraining force

determines how the material flows and consequently the stress state in the

sheet. When inplane compressive stress exists in the sheet, wrinkling could

initiate in the frustum region where the blank is free of normal constraint or in

the flange area where a pressure is imposed onto the sheet by the blank

holder.Hence blank holding force is the important process parameter

governing the wrinkle initiation in the deep drawing process, similarly corner

radii and lubrication regimes also play a considerable role on wrinkles.

Wrinkling study aims in identifying the significant parameters

responsible for flange wrinkling and optimizing the same. The number of

buckling waves found in the flange region during screening experiments was

taken for use in the analysis of variance. This study was also carried out at the

maximum LDR of the conventional deep drawing process.

The screening experiments remained the same as in previous study.

Before sectioning the cup for unveiling the thickness distribution the number

of buckling waves in the flange region in each of the formed cup was

measured using a microscope. Analysis of variance was performed and the

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significant variables affecting the wrinkling were listed out. Full factorial

experiments were conducted by varying the significant variables at multiple

levels and the optimum values of the significant parameters were found.

3.3.1.4 Maximum draw force estimates

It is always desirable to have lower draw force. Higher the draw force,

greater is the amount of wear on the tooling. This is critical in industry where

expensive tooling for complicated components cannot be replaced on a

regular basis. Also by reducing the draw force, for the same component lower

capacity press can be selected. The screening experiments remained the same

as in the previous studies. The maximum draw force value measured during

drawing was used for ANOVA. The significant variables responsible for

lower draw force were listed out and the optimum values were found through

full factorial experiments.

3.3.2 Deep drawing with MoS2 lubrication

All the experiments remained same as conventional deep drawing

process, the only difference being; instead of using fluid lubricant to lubricate

the interfaces, the powder lubricant Molybdenum disulphide powder was

wiped onto surfaces. For studies on maximum LDR Molykote Z powder used

due to less coefficient of friction and for the other studies both Molykote Z

and Molykote micro size powders were used.

3.3.3 Deep drawing with hydrostatic lubrication

These studies were associated with fluid pressure assisted blank

holding; initially maximum LDR for the same material through this process

was estimated. Later significant parameters are listed out and optimized with

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respect to thinning, flange wrinkling and draw force requirements. This was

done at the maximum limiting draw ratio of the conventional process thereby

aiding for the comparison of both the processes.

3.3.3.1 Process sequence

The die block was assembled with the required die insert and locating

ring and was placed over the bottom platen. The punch and die block was

checked for the eccentricity and were aligned. The blank was placed over the

surface of the die block and was held in position by the locating ring. In this

case the boundary lubrication condition prevails in the bottom surface and

hence the lubricant was wiped on to the die side of the blank

For the process with hydrostatic pressure assisted blank holding, the

pressure pad drilled with circumferential holes was used. The pressure pad

was fitted to the blank holding unit and the lubricant container was filled with

the lubricant. Here initially the punch slide was brought down and the blank

was held in position by punch. Then the blank holding slide was brought

down and it was stopped just above the blank without touching it.

The lubricant passages in the shaft were opened and the lubricant

flowed through the holes of the shaft to the cavity on the upper surface of the

pressure pad. From the cavity, lubricant was supplied to the blank holder

interface through circumferential holes. Till the lubricant touches the blank

surface, the flow was directed only by gravity. After ascertaining that the

lubricant has touched the blank surface the external power pack for

pressurizing the lubricant was switched on. The blank holder was brought

down and made to touch the blank surface and the hydrostatic pressure started

mounting up in the enclosed volume. The time for the required pressure to

build up was calculated before the actual process and the drawing was started

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after that stipulated time. The punch stroke and the velocity were fixed prior

to the process.

The draw force through out the stroke was measured with a pressure

transducer mounted on the fluid line of the main cylinder. The values could be

later plotted with respect to the punch stroke. The punch velocity and the

stroke could be captured online using LVDT. Once the cup was drawn

successfully, the punch was retracted and the pressure pad was lifted. The cup

was ejected using ejector fitted to the die.

3.3.3.2 Maximum LDR

The studies were aimed in estimating the maximum limiting draw ratio

of the process with hydrostatic pressure assisted blank holding. The vital

process parameters are listed below

1. Punch corner radius

2. Die corner radius

3. Blank holding force

4. Punch velocity

5. Radial distance of the circumferential holes

6. Lubricant viscosity

Out of six variables listed, the first four variables directly influence the

LDR and the last two variables indirectly influence the LDR but have a

considerable effect on the hydrostatic pressure distribution. Table 3.12 gives

ranges of various process variables that could be applied with the designed

experimental setup. The table also lists the values used when estimating the

maximum limiting draw ratio.

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Table 3.12 Process variables used for estimating maximum LDR - Process with hydrostatic lubrication

Process variable Range Used

Punch corner radius 3,5,7 mm 7 mm

Die corner radius 3,5,7 mm 7 mm

Punch velocity 1-35 mm/sec 10 mm/sec

Blank holding force 20 tons Calculated using Equation 3.2

Punch-die clearance 1.58 mm 1.58 mm

Lubricant CH 401, CH 402 CH402

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Some assumptions have been made by analyzing the pressure

generation problem theoretically. The analysis clearly predicts that for the

required pressure generation, the volumetric flow rate needed is less when the

viscosity of the lubricant is high. Hence lubricant with high viscosity is used

while estimating the maximum limiting draw ratio. The limiting draw depth is

calculated by Equation 3.1 and the punch stroke is kept below the limiting

draw depth.

The blank holding force was varied within the range and the cup was

drawn. The required LDR is said to be achieved when the cup is drawn

without failures. For this process it was decided to start with the maximum

LDR that was achieved with conventional deep drawing process. The

datasheet prepared to acquire data for this purpose during experimentation is

given in Figure 3.6.

As done before for the conventional deep drawing process, the failure

modes during drawing were observed and the punch stroke during failure was

measured. If the failure was due to excessive thinning and fracture then the

magnitude of the blank holding force was reduced and if the failure was due

to flange wrinkling then the magnitude of the blank holding force was

increased. The experiments were repeated again for lesser or higher blank

holding force and for the corresponding hydrostatic pressure.


The thinning studies performed were similar to that performed in the

conventional deep drawing process, the only difference being the process

variables. Lubricant position had been replaced with radial distance of the

circumferential holes.

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SAMPLE

Punch corner radius - 7 mm Punch- die clearance

Die corner radius - 7 mm Punch velocity

Radial distance of circumferential holes - 35 mm Lubricant used

Expt.

No

Blank holding

force (KN)Sample No. Punch stroke at failure Maximum punch

forceThinning Wrinkling

1 Minimum

1

2

3

2 Mean

1

2

3

3 Maximum

1

2

3

Figure 3.6 Sample data sheet for maximum LDR estimation - Process

with hydrostatic lubrication

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Except the lubricant type all other variables were continuous variables

and hence minimum and maximum in the range were chosen. Lubricant type

was discrete and had two levels. The total number of factors being 7 and each

had two levels; it is decided to use the same Taguchis L8 orthogonal array.

The significant variables would be found using analysis of variance

based on percentage of significance. Keeping all other factors constant full

factorial experiments were conducted by varying the significant parameters.

The optimum values of the significant parameters were chosen for the

selected limiting draw ratio. The thinning studies were performed at the

maximum LDR achieved through conventional deep drawing process. Table

3.13 shows the factors for experiments and Table 3.14 shows the levels of

factors that have been decided upon to use.

3.3.3.4 Flange wrinkling studies and maximum draw force estimates

The wrinkling studies and maximum draw force estimates were similar

to that of conventional process. L8 orthogonal array given in Table 3.13 was

used for screening the significant variables. The number of buckling waves

was used for ANOVA for flange wrinkling studies and maximum draw force

measured during drawing was used for draw force estimates.

3.3.4 Deep drawing with hydraulic counter pressure

This process was quite similar to that of the processes discussed in the

previous part. The only difference was that the die cavity was filled with the

lubricant. Hence, when the drawing progresses into the die cavity the pressure

in the die cavity was also increased. The pressure was maintained below the

threshold pressure so that the lubrication condition in the blank-blank holder

interface and blank-die interface were not disturbed.

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Table 3.12 L8 Orthogonal array with factors - Process with hydrostatic lubrication

Expt. No.

Factor number

1 2 3 4 5 6 7

Punch corner

radius (mm)

Die corner

radius (mm)

Blank holder

force (N)

Lubricant

type

Radial

distance of

holes (mm)

Punch

velocity

mm/sec

Draw depth

(mm)

1 1 1 1 1 1 1 1

2 1 1 1 2 2 2 2

3 1 2 2 1 1 2 2

4 1 2 2 2 2 1 1

5 2 1 2 1 2 1 2

6 2 1 2 2 1 2 1

7 2 2 1 1 2 2 1

8 2 2 1 2 1 1 2

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Table 3.13 Levels of factors Process with hydrostatic lubrication

Punch corner

radius (mm)

Die corner

radius (mm)

Blank holder

force (N)

Lubricant

type

Radial

distance of

holes

Punch

velocity

mm/sec

Draw depth

1 Low 3 3 a* CH 401 30 mm 5 c*

2 High 7 7 b* CH 402 35 mm 15 d*

a* - lower range of blank holding force calculated with Equation 3.2 for the given LDR

b* - higher range of blank holding force calculated with Equation 3.2 for the given LDR

c* - 50% of the limiting draw depth calculated using Equation 3.1 for the given LDR

d* - 75% of the limiting draw depth calculated using Equation 3.1 for the given LDR

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The threshold pressure is given by Lang et al (2000) as

( )( )

( )ddd

ddbh

dd

p

srRr

ts

rRRmF

rR

RktR

p+

++

+=

2

2ln22

(3.4)

c

cp

spsV

HERpp

2+= (3.5)

Since the frictional state in the interfaces did not differ much from the

previous process, it was assumed that the maximum LDR would remain same

with hydraulic counter pressure.


In addition to the variables in the previous processes, the one which

was added in the present process was the pressure in the die cavity. For the

screening experiments the parameter punch stroke was replaced with thepressure in the die cavity. The drawn cup was sectioned and the thickness was

unveiled and the minimum thickness measured is used for ANOVA.

3.3.4.2 Wrinkling studies and maximum draw force estimates

These studies were similar to that discussed earlier. After ANOVA full

factorial experiments were conducted to optimize the parameters.

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CHAPTER_III - Blank Holder Force